1. Field of the Invention
This invention relates generally to gas discharge light sources and the applications of those devices, including for the production of ultra-pure water such as used in semiconductor processing. This invention also relates to an excimer gas discharge light source for producing high intensity UV and Vacuum UV light.
2. Description of the Related Art
Volatile organic compounds and other organic chemicals are widely used as solvents, degreasers, coolants, gasoline additives, and raw materials for other synthetic organic chemicals. These organic compounds are commonly found as trace contaminants in municipal and natural water streams. As a group, they are referred to as total oxidizable carbons (TOC). These compounds are very difficult to remove by conventional means, such as filtration and absorption by media such as activated carbon.
A number of methods have been developed to remove TOC from water for applications requiring ultra-pure water. These methods physically separate the TOC from the water, chemically bind them so they are removed from the water, or chemically break them down into harmless components.
Physical separation is usually performed through a distillation process. This is an effective process, but is expensive and has limitations on throughput. It also creates a disposal problem, because the TOC are not destroyed in the process.
Chemical binding is normally performed by introducing activated carbon into the water, which leads to a chemical reaction that removes the TOC. Chemical breakdown of the TOC can be performed by catalysts, for example. The effectiveness of catalytic reactions is very dependent on the contaminant. TOC usually are not completely broken down by catalysts, and the introduction of the catalyst may lead to other problems in ultra-pure water systems.
Exposure to ultraviolet light is another means of removing TOC from water in ultra-pure water systems. The ultraviolet light for TOC removal in current commercially available systems is produced by low-pressure mercury vapor lamps operating at the 185 nm wavelength. There also exist systems using pulsed light sources that produce broad spectrum light below 250 nm. These pulsed light sources are typically xenon flashlamps. Excited dimer (“excimer”) pulsed discharge lamps have also been employed for removing TOC.
There are problems with the use of each of these lamps. For pulsed flashlamps, the conversion efficiency of input energy to light is less than 50%, only a small fraction of which is useful for removing TOC. For conventional excimer lamps, this figure is much lower, typically less than 5%. Direct discharge excimer lamps are limited to pulse lengths of about 100 ns, with a 10's of μs recovery time between pulses. This severely limits the energy throughput of the system and requires complicated electronics to achieve optimal performance. Pulsed flashlamps suffer from the same difficulty, albeit on longer time scales. This results in further degradation of process efficiency, and an even higher process cost. Pulsed flashlamps produce broadband radiation that would seem to overcome this limitation, but the blackbody nature of the spectrum generated by pulsed flashlamps still leads to generation of a large amount of the light at unproductive wavelengths. The result of all these inefficiencies is a process that is very expensive both in initial and operating cost. These technologies are not commonly used for that reason.
Conventional ultraviolet (UV) systems for TOC reduction normally use low or medium pressure mercury vapor lamps. These lamps are similar to common fluorescent lamps, but use higher quality components and a different operating point to radiate UV light with the proper spectrum for destroying TOC's. These lamps convert electrical energy into TOC-destroying ultraviolet light with 2.5% to 5% efficiency. A major drawback of these systems is the presence of mercury, which is a contamination concern in the event of lamp breakage.
The low conversion efficiency in producing the desired 185 nm light and the narrow bandwidth of that light in mercury vapor lamps leads to systems which are physically large and which require a large number of lamps to achieve the desired level of TOC removal in ultra-pure water systems. This leads to high initial and operating costs due to floor space and lamp replacement requirements.